1. Field of Invention
The field of the current invention relates to an apparatus, method and system for exposing a region of interest of an object, animal or person to an alternating magnetic field.
2. Discussion of Related Art
The use of radiofrequency (RF) electromagnetic fields has many applications in biology and medicine, for example. Among these are hyperthermia treatments for cancer, and other diseases and ailments, and for imaging tissues such as in magnetic resonance imaging (MRI). RF fields used for hyperthermia cancer treatment and high-field magnetic resonance imaging expose tissues to high amplitude fields with frequencies of 50 kHz-20 MHz for a period of time. For hyperthermia treatment the intent is to selectively heat cancer tissue to damage and kill cancer cells, or to sensitize cancer cells to the effects of radiation and anti-cancer drugs. Alternating magnetic fields (AMF) in the radiofrequency spectrum can be used to localize heat by heating antigen-targeted magnetic nanoparticles in the cancer tissue. For MRI, the tissue exposure of RF results from and depends upon the nature of activation of the imaging sequences of high field MRI devices. In both cases, direct tissue heating results from interaction of AMF with tissue. For cancer hyperthermia the challenge is to minimize this non-specific power deposition over large regions of tissue to avoid overheating and damaging or killing normal surrounding tissue.
It has been established that the application of heat for cancer therapy has significant potential, particularly when used in combination with radiation. The profound effect of heat on cancer cells is largely due to the physical environment of the tumor, and not necessarily because cancer cells are intrinsically more sensitive to heat. However, cancer cells generally possess more limited recovery capabilities than their normal counterparts contributing to the overall increased susceptibility to heat. Chronic hypoxia, low pH, chaotic vascularity, and nutritional deprivation characterize the interior of many tumors that consequently increases the sensitivity of cells to heat. In addition, mammalian cells are most sensitive to effects of heat and radiation at different stages of the mitotic cycle, further enhancing the potential therapeutic effects of the combination. (M. R. Horsman, J. Overgarrd; Hyperthermia: a potent enhancer of radiotherapy, Clinical Oncology (2007) 19, pp. 418-426; M. W. Dewhirst, E. Jones, T. Samulski, Z. Vujaskovic, C. Li, L. Prosnitz in Cancer Medicine, D. W. Kufe, R. E. Pollock, R. R. Weichselbaum, R. C. Bast, Jr., T. S. Gansler, Eds., (B C Decker, Hamilton, ed. 6, (2003) pp. 623-636. (sixth edition); J. L. Roti Roti, Heat-induced alterations of nuclear protein associations and their effects on DNA repair and replication, Int. J. Hypertherimia, (2008) 23, pp. 3-15; C. R. Hunt, R. K. Pandita, A. Laszlo, et al., Hyperthermia activates a subset of ataxia-telangiectasia mutated effectors independent of DNA strand breaks and heat shock protein 70 status, Cancer Res., (2007) 67, pp. 3010-3017).
It is believed that the beneficial effects of hyperthermia for cancer can only be realized if a therapeutic temperature (42° C. to 46° C.) is achieved and maintained for a sufficient period of time throughout the tumor (Dewhirst et al., Supra; M. W. Dewhirst, B. L. Viglianti, M. Lora-Michiels, M. Hanson, P. J. Hoopes, Intl. J. Hyperthermia (2003) 19, pp. 267-294). Generally, higher temperatures produce greater effects. Similar effects are observed with increased time of exposure. These requirements present technical challenges because the heating must occur while simultaneously minimizing heat deposition to the surrounding normal tissue. The combination of these challenges and other factors has inhibited widespread application of this tool in the clinical setting (Dewhirst et al., Cancer Medicine, Supra; J. van der Zee, Heating the patient: A promising approach? Annals of Oncology 2002, 13, 1173-1184).
These barriers become particularly challenging when attempting to address metastatic cancer, such as metastatic prostate cancer. Characteristic of metastatic disease is the widespread appearance of deep-tissue (>7 cm) tumors in many organs and bone. Many techniques and devices that have been developed to heat tissue deliver electromagnetic (“EM”) radiation in the radio- or microwave frequencies to a selected region of the body. The heat dose thus depends upon the interaction of time-varying electromagnetic fields with tissues, and upon the time of treatment. The manner of the tissue-EM interaction and resulting power or heat deposition from electromagnetic fields strongly depends upon the frequency of the EM field, and the dielectric permittivity and electrical conductivity of tissue(s) (C. Polk, Introduction in Biological and Medical Aspects of Electromagnetic Fields, Third Edition; Eds. F. S. Barnes and B. Greenebaum, CRC Press, Taylor & Francis Group, Boca Raton, Fla., (2006) pp. xiii-xxvi; U. Cerchiari, Hyperthermia, physics, vector potential, electromagnetic heating: A primer in Hyperthermia in Cancer Treatment: A primer, Eds. G. F. Baronzio and E. D. Hager, Landes Bioscience and Springer, New York, N.Y. (2006), pp. 3-18; A. Szasz, O. Szasz, N. Szasz, Physical background and technical realizations of hyperthermia in Hyperthermia in Cancer Treatment: A primer, Eds. G. F. Baronzio and E. D. Hager, Landes Bioscience and Springer, New York, N.Y. (2006), pp. 27-52; E. R. Adair, D. R. Black, Thermoregulatory responses to RF energy aborption Bioelectromagnetics Supplement (2003), 6, S17-S38). In biological tissue with finite conductivity σ (S/m) the electric field Ē (V/m) induces a current, Ī=σĒ, that deposits power in the tissue via joule heating. The power deposited to the tissue is defined as the specific absorption rate (SAR) given by
                              S          ⁢                                          ⁢          A          ⁢                                          ⁢          R                =                              σ            ⁢                                                                            E                  →                                                            2                                            ρ                          +              n                                                          (        1        )            where, ρm is the mass density of tissue in (kg/m3) (Szasz, et al., supra; F. Liu, H. Zhao, S. Crozier, On the induced electric field gradients in the human body for magnetic stimulation by gradient coils in MRI, IEEE Trans. On Biomedical Engineering (2003) 50, pp. 804-815).
It is difficult to control this heating because living tissue is a highly complex and responsive medium that contains layers of tissue differing in composition and density, comprises a fractal network of blood vessels that transport heat, and harbors multiple interfacial regions that can reflect, scatter, or absorb EM waves. Reflection and transmission of EM waves impinging a planar tissue interface are determined by the properties of the EM waves and tissue, and by the geometry of the interaction (M. J. Hagman, O. P. Gandhi, C. H. Durney, Numerical calculation of electromagnetic energy deposition for a realistic model of man, IEEE Trans. Microwave Theory and Technol., (1979) 27, p 804; Liu, et al., supra). These effects place demands on technological innovation thus limiting the successful translation into clinical settings.
High frequency electric fields are strongly attenuated by soft tissue and bone, the latter having a particularly strong effect. Conversely, low frequency magnetic fields, i.e. 100-300 kHz, are not significantly attenuated by tissue, even bone, making this an attractive mode for metastatic disease therapy. Low frequency fields heat tissue by capacitive and magnetic induction; however, they cannot discriminate normal tissue from tumors (Szasz, et al., supra).
Coupling low-frequency energy with magnetic materials concentrated in the target region offers more precise and selective heat deposition by providing a susceptive material in the cancer tissue that efficiently couples with the electromagnetic energy. And, this approach simultaneously avoids the interference to power absorption caused by bone and boundaries occurring at the interface of tissues with varying conductivities (A. Candeo, F. Dughiero, Numerical FEM models for the planning of magnetic induction hyperthermia treatments with nanoparticles, IEEE Trans. on Magnetics (2009) 45, pp. 1658-1661; A. Jordan, P. Wust, R. Scholz, H. Faehling, J. Krause, R. Felix, Magnetic fluid hyperthermia (MFH), Scientific and Clinical Applications of Magnetic Carriers, Eds. U. Häfeli, M. Zborowski, W. Schütt, Plenum Press, New York, N.Y. (1997), pp. 569-595). Recent efforts have demonstrated the potential to selectively heat tumors with biocompatible magnetic nanoparticle suspensions or ferro-fluids exposed to external alternating magnetic fields (S. J. DeNardo, G. L. DeNardo, A. Natarajan, L. A. Miers, A. R. Foreman, C. Gruettner, G. N. Adamson, R. Ivkov, Thermal dosimetry predictive of efficacy of 111In-ChL6 nanoparticle AMF-induced thermoablative therapy for human breast cancer in mice, J. Nuc. Med., (2007) 48, pp. 437-444). Induction heating techniques and equipment are most applicable for this approach to thermal therapy, with particular emphasis on the design and manufacture of induction coils (P. R. Stauffer, P. K. Sneed, H. Hashemi, T. L. Phillips, Practical induction heating coil designs for clinical hyperthermia with ferromagnetic implants, IEEE Transactions on Biomedical Engineering, (1994) 41, pp. 17-28).
The magnetic particles are nano-scale susceptors that generate heat via several potential mechanisms when exposed to the externally applied AMF (C. L. Dennis, A. J. Jackson, J. A. Borchers, P. J. Hoopes, R. Strawbridge, A. R. Foreman, J. van Lierop, C. Grüttner, R. Ivkov, Nearly complete regression of tumors via collective behavior of magnetic nanoparticles in hyperthermia, Nanotechnology (2009) 20, pp. 395103; R. E. Rosensweig, Heating magnetic fluid with alternating magnetic field, J. Magnetism and Magn. Materials, (2002) 252, pp. 370-374; Jordan, et al., supra). The amount of heat deposited in a volume of tissue, D (Joules/g tissue) at fixed frequency, is proportional to the tissue particle concentration, Π (g particle/g tissue), the AMF-amplitude dependant power loss function of the particles, Ψ(H) (Watts/g particle), the amplitude of magnetic field, H (Amperes/m), and total treatment time, t (seconds) (DeNardo, et al., supra):D=Π×Ψ(H)×H×t.  (2)
Additional heat may result from the magnetic dipole interactions of the particles that result from an inhomogeneous distribution within the tumor and around cells (Dennis, et al., supra). The additional heating resulting from this mechanism may contribute significantly to the potential success of this treatment (Dennis, et al., supra; Candeo, et al., supra).
The characteristics of low-frequency EM field interactions with the biological tissue are: (1) the E-fields are concentrated at the surface of the biological body, and perpendicular to the surface, (2) E- and H-fields are decoupled inside a biological body, (3) the E-fields are weakened by a large dielectric permittivity upon penetration into biological tissues, (4) the magnetically induced E-field encircles the H-field and produces eddy currents with a magnitude that increases with distance from the center of the body, and (5) an eddy current appears in each region inside the body with a different conductivity and behaves as a unit with its own body center and radius or equivalent radius (F. S. Barnes, B. Greenbaum, Bioengineering and Biophysical Aspects of EM Fields, pp. 298-297; Liu, et al., supra; A. Trakic, F. Liu, S. Crozier, Transient temperature rise in a mouse due to low-frequency regional hyperthermia, Phys. Med. and Biol., (2006) 51, pp. 1673-1691).
Interactions of low-frequency E-fields with magnetic nanoparticles provide little contribution to therapeutic heating effects, and thus are considered irrelevant in the current context. In contrast, optimizing the B-field interaction with magnetic particles by increasing both amplitude and improving the geometry of interaction is desired to maximize the heat produced in the target tissue. However, non-specific heat is also deposited via tissue coupling with the B-field component of EM fields through magnetic induction, or production of eddy currents. The heat deposited, or the SAR, by this process is proportional to tissue conductivity, and EM parameters:SAR∝σ×f2×H2×r2,  (3)where f is frequency of AMF in Herz (Hz), and r is radius of the exposed tissue region. Increasing the heat dose deposited by the particles at constant frequency for a given particle concentration can be accomplished by increasing the AMF amplitude (Equation 2), which also increases eddy current heating and SAR (Equation 3) (R. Ivkov, S. J. DeNardo, W. Daum, A. R. Foreman, R. C. Goldstein, V. S. Nemkov, G. L. DeNardo, Application of high amplitude alternating magnetic fields for heat induction of nanoparticles localized in cancer, Clin. Cancer Res. (2005) 11(19 Suppl), pp. 7093s-7103s). Therefore, it becomes a challenging compromise to maximize the heat output of the particles by increasing the AMF amplitude homogeneously over a large region of interest (ROI) at a select frequency of AMF without depositing excessive power in normal tissue. There thus remains a need for improved systems to expose ROI's of an object to high amplitude AMF while decreasing the amount of heating in regions around the ROI's.